Electronic timer

ABSTRACT

The invention relates to an electronic timer using a constant current discharge of a capacitor. This discharge is effected through the main electrodes of a transistor. According to the invention a linear potentiometer is incorporated in a circuit between the emitter and the base of the transistor, and the wiper of this potentiometer is connected to both the capacitor and to one of the input terminals of the device. In this embodiment it is achieved that the adjusting range of the switching time is relatively large, and that also the percentual accuracy is substantially the same throughout the adjusting range.

United States Patent [191 Luursema ELECTRONIC TIMER [75] Inventor: Meerten Luursema, Emmasingel,

Eindhoven, Netherlands [73] Assignee: U.S. Philips Corporation, New

York, N.Y.

[22] Filed: Sept. 11, 1972 21 Appl. No.: 288,140

[30] Foreign Application Priority Data Sept. ll, 1971 Netherlands.. 7112528 US. Cl. 307/141, 317/142 R Int. Cl. H01h 3/34, HOlh 33/59 Field of Search 307/132 E, 141, 141.4; 315/60; 317/142 R [56] References Cited UNITED STATES PATENTS 2.818.532 12/1957 Aitel ..3l7/l42R Mar. 19, 1974 2,434,157 l/l948 Heppeard 3l5/360 Primary Examiner-David Smith, Jr. Attorney, Agent, or Firm-Frank R. Trifari [5 7 ABSTRACT The invention relates to an electronic timer using a constant current discharge of a capacitor. This discharge is effected through the main electrodes of a transistor.

7 Claims, 3 Drawing Figures ELECTRONIC TIMER The invention relates to an electronic timer which is provided with two input terminals intended to be connected to a direct voltage source, which input terminals are connected by means of a potential divider a tap of which is connected to the base of a transistor. The main electrodes of the transistor are incorporated in a close circuit in series with a capacitor and at least one resistor, said resistor and part of the potential divider being also located in a connection from the emitter of the transistor to the tap. The capacitor is coupled to a switching device and the current flowing through the closed circuit has a substantially constant value during operation of the timer.

A known timer of the kind mentioned above is described, for example, in Netherlands Pat No. 69101 14. When the capacitor is completely discharged, the said switching device is operated thereby.

When this known timer is to be set to a different switching time, the capacitor is to be given a different initial. voltage. The discharge of the capacitor is effected with a non-variable constant current intensity during operation of thistimer. A drawback thereof is that even for a range (namely between the longest switching period and the shortest switching period) of approximately :1, high initial'voltages may be present across the capacitor. In the case of still larger ranges, change-over devices have to be added to the arrangement. This is also a drawback.

When the object is to provide a simple electronic timerwhich, in addition to the requirement of a wide range, for example, 40:1, also satisfies the condition that within this adjusting range the same small percentual adjusting accuracy is achieved throughout, the use of inter alia a logarithmic potentiometer is considered. Such logarithmic potentiometers are, however, generally too inaccurate for the envisaged object.

It has surprisingly been found that in spite of the requirement of v a small percentual adjusting accuracy throughout the range, the envisaged object can be achieved with the aid of a simple linear potentiometer.

, According to the invention an electronic timer is provided with two input terminals which are adapted to be connected to a direct voltage source. The input-terminals'are connected to a potential divider having a tap which is connected to the base of a transistor. The main electrodes of the transistor are incorporated in a closed circuit in serieswith a capacitor and at least one resistor. The said resistor and part of the potential divider are alsolocated in a connection from the emitter of the transistor to the tap, while the capacitor is coupled to a switching device and the current flowing through the closed circuit has a substantially constant value during operation .of the timer. A characteristic feature of the invention isethatat least part of the resistor, together with a part of the potential divider located in the connection from the emitter of the transistor to the tap, is formed as a linear potentiometer whose wiper is connected to one of the input terminals and also forms part of the closed circuit. The largest ratio between the resistive value of the part of the potential divider between the wiper and the tap and that of the part of the potential divider at the other end of said tap is 1.3 at a maximum. A

An advantage of this timer is that the switching time can be adjusted in a simple manner and with a percentual accuracy which is substantially the same throughout a relatively large range. This adjustment is effected by moving the wiper along the potentiometer resistor. The operation of the switching device at the end of the adjusted switching time is effected, for example, by the discharged capacitor or in more general terms: by such a discharged condition of the capacitor that only a low voltage is present across this capacitor (after the constant current discharge), or by reaching a relatively high capacitor voltage (after a constant current charge of the capacitor).

The typical adjustment of the switching time in a timer according to the invention is related .to the fact that simultaneously the resistance in the closed circuit (including the capacitor and the main electrodes of the transistor) and the potential divider ratio of the potential divider connected across the input terminals is varied by moving the wiper on the potentiometer. This is effected in a switching device according to the invention in such a manner that a decrease of the resistance R in the closed circuit (the emitter resistor) is accompanied'by an increase of the voltage E across this resistor (as a-result of the larger voltage due to the varied voltage division between the base of the transistor and the wiper on the potentiometer). This of course results in a strong increase of the current (E/R) in the closed circuit and hence a much faster charging or discharging of the capacitor and consequently a much shorter switching time. A small range of the linear potentiometer thus already results in a large range of theswitching time. I

The ideal relation between the position y of the wiper on the potentiometer and the current i through the closed circuit maybe represented in principle by y the expression =C log i. C2, in which C1 and C represents constants. Due to theabove-mentioned E, R influence this ideal relation can be satisfactorily approximated with a timer according to the invention as willbe described hereinafter...

The above-mentioned requirement, i.e. that the largest ratio of the resistive value between the part of the potential divider between the wiper and the tap and that of the part of the potential divider at the other end of this tap is 1.3at a maximum, is related to the fact that in that case a moderate variation of the position'of the wiper also leads to an acceptable, i.e., not too small, variation of the voltage between wiper and tap (or between the wiper and the base of the transistor).

It is possible to provide the linear potentiometer with two stopsbetween which the wiper can be moved. In that case the resistance in the closed circuit can be prevented from becoming too low and the voltage between the base and the emitter of the transistor can be prevented from becoming zero (no main electrode current).

.In a preferred embodiment of an electronic timer according to the invention, the connection between the emitter and the base of the transistor consists of, successively, a first resistor, the total resistor of the linear potentiometer and a second resistor, in which the first resistor and the second resistor are substantially equal and have a value of between 1/7 and 1/10 of the total resistance of the linear potentiometer.

An advantage thereof is that a linear potentiometer of lower value may be sufficient, and moreover the wiper can cover the entire potentiometer resistor tra jectory.

With the given relation between the resistive value of the two resistors and that of the potentiometer, all located between the base and the emitter of the transistor, the control range is prevented from extending into extreme marginal ranges of the capacitor current adjustment.

In a further preferred embodiment the largest ratio between the part of the potential divider between the wiper and the tap and that of the potential divider at the other end of this tap is approximately one.

This further preferred embodiment has the advantage that a shift in equal steps of the wiper leads to a very satisfactory exponential variation of the capacitor current so that a scale along the wiper can be given a logarithmic character.

As is known the percentual adjusting accuracy is the same throughout the meauring range of such a scale so that in this case the requirement referred to in the preamble is well satisfied.

The electronic timer is used, for example, for switching on or switching off an electric appliance after a given period, for example, an electrical kitchen range. The switching device of the timer then operates, for example, a work contact in the supply of the range.

A further application is-in a device for feeding an exposure ligh source of a photographic enlarger. This is an interesting application because in such an enlarger the exposure time must generally be varied in a large range with an adjusting accuracy which is the same throughout.

The invention will be further described with reference to the accompanying drawing. In this drawing:

FIG. 1 shows an electrical principle circuit diagram of an electronic timer according to the invention;

FIG. 2 shows a second electrical principle circuit diagram of an electronic timer according to the invention; and

FIG. 3 shows the magnitude of the capacitor current in the timer shown in FIG. 1, plotted against the position a of the wiper on the potentiometer.

In FIG. 1, connection terminals 1 and 2 are connection adapted to be connected to a direct voltage source (not shown). Terminal 1 is connected to the positive terminal of this source and terminal 2 is connected to its negative terminal. Terminal 1 is connected to a switch 3. Switch 3 is connected to a potential divider consisting at one end of resistor 4 and at the other end of the combination of a resistor 5 and a part 7 of a linear potentiometer 8 located between the wiper 6 and the resistor 5. Wiper 6 is connected to the negative terminal 2. The other part of the potentiometer is denoted by reference numeral 9. A tap 10 between the resistors 4 and 5 is connected to the base 11 of a transistor 12. This transistor is of the npn-type. The emitter of transistor 12 is connected through a resistor 13 to the part 9 of potentiometer 8. A Capacitor 14 is connected, in series with a switch 15, to the collector of transistor 12. The other end of capacitor 14 is connected to the wiper 6 of potentiometer 8.

In addition capacitor 14 is incorporated in a further circuit which serves to charge this capacitor. This circuit consists of two connection terminals 17 and 18 which connect a two-pole switch 19 and a resistor 20 to a direct voltage source. In the closed condition of switch 19 capacitor 14 is charged through resistor 20 from the direct voltage source mentioned above.

Furthermore a voltage-sensitive circuit represented by block 16 is connected across the series arrangement of capacitor 14 and switch 15. This voltage-sensitive circuit is formed in such a manner that when the voltage across capacitor 14 (in the closed condition of switch 15) is above a given value, an energizing winding 16a in block 16 conveys current and in which below this value of the voltage across capacitor 14 the energizing winding 16a does not convey current.

The contact of the relay whose energizing winding is denoted by 16a is denoted by reference numeral 21. This contact is included in series with a lighting lamp of a photographic enlarger 22, namely between two input terminals 23 and 24 which are intended to be connected to an alternating voltage supply of, for example, 220 Volt 50 l-lz. Part 25 represents a photographic negative and 26 is a photosensitive plate onto which the enlarged negative image is projected.

In one embodiment the voltage of the direct voltage source between terminals 1 and 2 was approximately 24 Volts. The resistor 4 had a value of approximately 560kOhm. The resistor 5 had a value which was approximately equal to that of resistor 13, namely approximately 58 kOhm. The total resistance of potentiometer 8 was approximately 470 kOhm. The ratio R8 +R5/R4=47O+58/560 was therefore approximately 1. The ratio R13 /R8=58/470 is approximately oneeighth, ie it was between one-seventh and one-tenth. Transistor 12 was a silicon transistor whose voltage in the conducting state between the base and the emitter was very low. The capacitor 14 had a capacitance of approximately 10 micro-Farad. This capacitor was always charged to an initial voltage of approximately 65 Volts, namely through resistor 20. The threshold voltage of the arrangement 16 was approximately 15 Volts.

By moving the wiper 6 on the potentiometer 8 the resistive value of part 7 of the potentiometer may be varied proportionally to the resistive value of part 9 of this potentiometer. In the extreme position in which the wiper was moved entirely to the left the voltage between point 10 and wiper 6 was low, namely R5/R5 R4 times the supply voltage. In that case the resistance in the closed circuit 14, 15, 12, 13, 9, 6, 14 had a large value. In fact, this resistance consisted of the resistor 13 plus the total resistance of potentiometer 8.

In the situation in which wiper 6 was moved entirely to the right, at which the resistor 9 had a value of zero and resistor 7 had a value which was equal to the total resistance of the potentiometer, the voltage between wiper 6 and the tap 10 was high, namely R5 R8/R4 R5 R8 times the supply voltage. On the other hand in this situation the resistance in the previously mentioned closed circuit 14, 15, 12, 13, 6, 14 had a low value because in this case this resistance was mainly constituted by resistor 13.

The said voltage between wiper 6 and tap 10 can also be considered with a satisfactory approximation as the voltage between the emitter of transistor 12 and wiper 6.

The ratio between the resistive part 7 of the potentiometer and the total resistor 7 9 of the potentiometer 8 will hereinafter be referred to by a.

The operation of the circuit of FIG. 1 is as follows. First, capacitor 14 is charged to a voltage of approximately 65 Volt by closing switch 19. Subsequently switch 19 is opened again and switch 3 is closed. The circuit is then ready to start exposure with the aid of lamp 22. To this end the wiper 6 moving along a logarithmic time scale is adjusted to the desired switching time. This time scale (t-scale) is shown enlarged in FIG.

.3 and will be described hereinafter. Switch is subsequently closed. This means that capacitor 14 starts to discharge at a constant current across the circuit constituted by switch 15, the main electrodes of transistor 12, resistor 13, part 9 of the potentiometer and wiper 6. Due to the high voltage across the arrangement 16 the energizing winding 16a will also start to convey current so that the work contact 21 closes and lamp 22 is ignited. When capacitor 14 is discharged to such an extent that the threshold voltage of arrangement 16 has been reached the current flowing through the energizing winding 16a will be interrupted and the work contact 21 will open so that the exposure light source 22 is extinguished. This is the end of the exposure time.

FIG. 3 shows, for the situation of FIG. 1, how the constant discharge current i (in micro-amperes) of capacitor 14 varies as a function of the position a of wiper 6. on the potentiometer, represented by the solid line curve.

The vertical axis of FIG. 3 is a logarithmic axis. The upper horizontal axis of this Figure is a linear axis. FIG. 3 furthermore shows by means of a broken line an ideal logarithmic variation between i and a. This broken line is of course a straight line. FIG. 3 shows that the deviation of the true curve relative to the straight line is relatively very small. The deviation is in the order of 15 percent. I

In the case of bulk manufacture of electronic timers of a type as herein described, such a deviation of the logarithmic scale is certainly admissible. If the said deviation exhibits, however, for all these apparatus the same undulating variation as is shown in FIG. 3, this can be included in the time scale along the wiper of the apparatus so that a precisely logarithmic scale cannot then be used but a scale which quite resembles this scale and which has the advantage that it corresponds to the actual exposure times.

It will be evident that a linear relationship is present between the capacitor current i and the exposure time of the light source. 22 because the capacitor current always maintains one constant value during one discharge. v

The t-sca'le in FIG. 3 having a logarithmic character "shows enlarged the time scale which is presentalongside the potentiometer 8 of FIG. 1. The Figures are expressed in a unit of time not further shown. A small a of course corresponds to a low discharge current i of capacitor 14 and hence to a long exposure time t.

In the relevant case a constant discharge of capacitor 14 took place using a transistor of the npn-type. It is likewise possible to use a transistor of the pnp-type in such a capacitor discharge. Alternatively it is possible that in a switching device according to the invention a constant current charge of the capaciator is used instead of the constant current discharge of the capacitor.

FIG. 2 shows an arrangement in which a constant capacitor current charge is used and in which a transistor of the pnp-type is used. The circuit of FIG.2 operates roughly the same as that of FIG. Input terminals 50 and 51 are intended to be connected to a direct voltage source in which terminal 50 is connected to the positive terminal. A switch 52 is connected to terminal 50. Switch 52 is furthermore connected to a wiper 53 on a linear potentiometer 54. One end of this potentiometer 54 is connected to a series arrangement of a resistor 55 and a resistor 56. The other end of resistor 56 is connected to the negative terminal 51. A tap 57 between resistors 55 and 56 is connected to the base 58 of a transistor 59. The other end of potentiometer 54 is connected through a resistor 55a to the emitter of transistor 59. The collector of transistor 59 is connected to a capacitor 60. The other end of capacitor 60 is connected to terminal 51. A series arrangement of a threshold element 61 and an energizing winding 62 of a switching device is connected in parallel across the capacitor 60. The switching device is not shown in further detail. The capacitor 60 is also shunted by a series arrangement of a resistor 63 and a switch 64. The operation of the circuit of FIG. 2 resembles that of FIG. 1. One difference is, however, that first the switch 64 is closed so that any possible charge on capacitor 60 is depleted. Then switch 64 is opened again and switch 52 is closed. As a result a current will flow through wiper 53 to resistor 55a, the main electrodes of transistor 59 and capacitor 60 to terminal 51. This current charges capacitor 60 at a constant current as a result of the voltage realized through the potential divider 55, 56 at the base 58. When the voltage across the capacitor thereby reaches a given value, the threshold element 61 will strike and will pass a current through the energizing winding 62 of a switching device which then operates a work contact, not shown. The value of the current through capacitor 60 can be adjusted by the position of the wiper 53 on potentiometer 54. After discharging capacitor 60, switch 64 is closed again so that any possible residual charge on capacitor 60 can be depleted and subsequently switch 64 is opened again and the switching device is ready for the next procedure.

The described switching devices according to the invention are simple and produce an adjustable discharge current (charge current) when using a linear-potentiometer (see 8 and 54), or an adjustable exposure time with which the relation between the shift of the wiper on the potentiometer relative to the variation in expo-v sure time has a logarithmic character so that substantially the same percentual accuracy is present throughout the adjusting range.

As FIG. 3 shows the range between the maximum capacitor current intensity and the minimum current intensity, or between the maximum exposure time and the minimum exposure time, is more than 40 in the described case. This is a relatively large range.

What is claimed is:

1. An electronic timer comprising two input terminals adapted to be connected to a direct voltage source, a potential divider, a transistor, a capacitor, means connecting the input terminals to said potential divider, means connecting a tap of the potential divider to the. base of the transistor, means connecting the transistor main electrodes in a closed circuit in series with the capacitor and at least one resistor, said resistor and a part of the potential divider being located in a connection between the emitter of the transistor and the potential divider tap, the capacitor being'coupled to a switching device and the current flowing through the closed circuit having a substantially constant value during operation of the timer, and wherein at least a part of the resistor together with a part of the potential divider located in the connection between the emitter of the transistor and the tap comprises a linear potentiometer having a wiper connected to one of the input terminals and as a part of the closed circuit, the largest ratio between the resistive value of the part of the potential divider between the wiper and the tap and that of the part of the potential divider at the other end of said tap being a maximum of 1.3.

2. An electronic timer as claimed in claim 1, wherein the connection between the emitter and the base of the transistor includes, successively, a first resistor, the total resistor of the linear potentiometer and a second resistor, the first resistor and the second resistor having substantially the same resistance value which lies between one-seventh and one-tenth of the total resistance of the linear potentiometer.

3. An electronic timer as claimed in claim 2, characterized in that the largest ratio between the part of the potential divider between the wiper and the tap and that of the potential divider at the other end of said tap is approximately one.

4. An electronic timer as claimed in claim 1, characterized in that a time scale having a mainly logarithmic division is present alongside the wiper on the potentiometer.

5. An electronic timer as claimed in claim 1 further comprising an exposure lamp for a photographic enlarger, voltage responsive means connected across the capacitor and including means for selectively applying a source of voltage to said lamp as a function of the capacitor voltage.

6. An electronic timer as claimed in claim 1 further comprising means connecting the potentiometer wiper to one electrode of the capacitor.

7. An electronic timer as claimed in claim 2 wherein 

1. An electronic timer comprising two input terminals adapted to be connected to a direct voltage source, a potential divider, a transistor, a capacitor, means connecting the input terminals to said potential divider, means connecting a tap of the potential divider to the base of the transistor, means connecting the transistor main electrodes in a closed circuit in series with the capacitor and at least one resistor, said resistor and a part of the potential divider being located in a connection between the emitter of the transistor and the potential divider tap, the capacitor being coupled to a switching device and the current flowing through the closed circuit having a substantially constant value during operation of the timer, and wherein at least a part of the resistor together with a part of the potential divider located in the connection between the emitter of the transistor and the tap comprises a linear potentiometer having a wiper connected to one of the input terminals and as a part of the closed circuit, the largest ratio between the resistive value of the part of the potential divider between the wiper and the tap and that of the part of the potential divider at the other end of said tap being a maximum of 1.3.
 2. An electronic timer as claimed in claim 1, wherein the connection between the emitter and the base of the transistor includes, successively, a first resistor, the total resistor of the linear potentiometer and a second resistor, the first resistor and the second resistor having substantially the same resistance value which lies between one-seventh and one-tenth of the total resistance of the linear potentiometer.
 3. An electronic timer as claimed in claim 2, characterized in that the largest ratio between the part of the potential divider between the wiper and the tap and that of the potential divider at the other end of said tap is approximately one.
 4. An electronic timer as claimed in claim 1, characterized in that a time scale having a mainly logarithmic division is present alongside the wiper on the potentiometer.
 5. An electronic timer as claimed in claim 1 further comprising an exposure lamp for a photographic enlarger, voltage responsive means connected across the capacitor and including means for selectively applying a source of voltage to said lamp as a function of the capacitor voltage.
 6. An electronic timer as claimed in claim 1 further comprising means connecting the potentiometer wiper to one electrode of the capaciTor.
 7. An electronic timer as claimed in claim 2 wherein said first resistor consists of said one resistor. 